1. Field of the Invention
The present invention relates to a projection optical system used in a color projector that applies illumination light to a transmissive liquid crystal panel or the like to project an image so as to be enlarged, and more particularly, to a projection optical system that reduces astigmatic difference caused when image information of R, image information of G and image information of B are combined.
2. Description of the Prior Art
A conventional projection optical system has an arrangement as shown in FIG. 1. Light beams of wavelength ranges of red, green and blue (hereinafter, referred to as "red light", "green light" and "blue light", respectively) separated by an illumination system (not shown) are applied to display panels 10R, 10G and 10B such as transmissive liquid crystal panels for displaying images of the colors. The display panels 10R, 10G and 10B transmit or reflect the applied light so that image information of the colors exits therefrom.
The green light exiting from the display panel 10G is transmitted by dichroic mirrors 4 and 5. The red light exiting from the display panel 10R is reflected at the dichroic mirror 4 and is transmitted by the dichroic mirror 5. The blue light exiting from the display panel 10B is reflected at the dichroic mirror 5. The red light, the green light and the blue light being combined are projected by a projection lens system 7 so as to be enlarged.
Since the dichroic mirrors 4 and 5 are inclined with respect to the light transmitted thereby, astigmatic difference is caused. The astigmatic difference increases as the inclination angle and the transmission thickness of a glass substrate or the like increases.
Means for correcting the astigmatic difference include the provision of a separate correction lens of a cylindrical configuration or the like that refracts only light of one of two orthogonal directions. However, in the case of FIG. 1, there is no astigmatic difference in the blue light, there is astigmatic difference due to the dichroic mirror 5 in the red light and there is astigmatic difference due to the dichroic mirrors 4 and 5 in the green light, so that the magnitude of the astigmatic difference is nonuniform. Therefore, it is difficult to correct the astigmatic difference by the correction lens.
A method of reducing the astigmatic difference is disclosed, for example, in Japanese Laid-open Patent Application No. H8-334727. According to this method, as shown in FIG. 2, the angles of inclination of the two dichroic mirrors 4 and 5 to the optical paths of the light transmitted by the dichroic mirrors 4 and 5 are reduced to thereby reduce the astigmatic difference.
Moreover, a method has been proposed in which, as shown in FIG. 3, the dichroic mirrors 4 and 5 having the same thickness are disposed at the same inclination angle in a so-called twisted position such that a plane including the incidence optical path R1 and the reflection optical path G1 of the red light reflected at the dichroic mirror 4 is orthogonal to a plane including the incidence optical path B1 and the reflection optical path G1 of the blue light reflected at the dichroic mirror 5 to thereby cancel out the astigmatic difference in the green light transmitted by the dichroic mirrors 4 and 5 and the image is projected onto a screen 30.
However, according to the method disclosed in Japanese Laid-open Patent Application No. H8-334727, since it is necessary to dispose the display panels 10B and 10R so as not to interfere with a luminous flux C for projection and the projection lens system 7 and that the optical path lengths of the luminous fluxes exiting from the three display panels 10B, 10R and 10G are the same, the lens back focal length LB of the projection system increases, so that the projection optical system increases in size.
According to the method shown in FIG. 3, although no astigmatic difference is caused in the blue light which is reflected at the surface of the dichroic mirror 5 and the astigmatic difference caused in the green light is canceled out by the green light being transmitted by the dichroic mirrors 4 and 5, astigmatic difference remains in the red light which is transmitted by the dichroic mirror 5. Further, when downsized display panels which have frequently been used in recent years are used, since an optical system close to a telecentric optical system is used in order to curb image deteriorations other than the astigmatic difference, the lens system increases in size, so that the cost increases.